1. Technical Field
The present disclosure relates to discharge tube lighting circuits for activating discharge tubes such as cold-cathode tubes. More particularly, the disclosure relates to a discharge tube lighting circuit for activating a U-shaped discharge tube or a pseudo U-shaped discharge tube (in the following, a pseudo U-shaped discharge tube is also merely referred to as a U-shaped discharge tube), and an electronic apparatus having a backlight provided with the discharge tube lighting circuit.
2. Background Art
Currently, cold-cathode tubes (discharge tubes) are used for backlights of large liquid crystal displays. Such discharge tubes include U-shaped discharge tubes and pseudo U-shaped discharge tubes. The U-shaped discharge tube is obtained by bending a single long discharge tube at a center portion thereof into a U-shape or an approximate U-shape. The pseudo U-shaped discharge tube is obtained by connecting two I-shaped or approximately I-shaped discharge tubes. In backlights of such displays, a plurality of U-shaped discharge tubes are usually disposed. For example, in the case of a backlight for a liquid crystal display with a 30 to 40-inch screen, over ten U-shaped discharge tubes are disposed.
In Japanese Unexamined Patent Application Publication No. 2005-5059 (JP '059), a discharge tube lighting circuit for activating U-shaped discharge tubes is disclosed.
Here, a simplified exemplary configuration of a discharge tube lighting circuit will be described with reference to the above-described document. FIG. 1 shows a discharge tube lighting circuit for activating two U-shaped discharge tubes. In a backlight 101, U-shaped discharge tubes 102A and 102B are connected to a discharge tube lighting circuit 105. The U-shaped discharge tube 102A is a pseudo U-shaped discharge tube obtained by connecting two I-shaped discharge tubes 103A and 103B. The U-shaped discharge tube 102B is a pseudo U-shaped discharge tube obtained by connecting two I-shaped discharge tubes 103C and 103D.
Power supply electrodes 104A to 104D for the I-shaped discharge tubes 103A to 103D are connected to resonant capacitors C1A to C1D included in the discharge tube lighting circuit 105 and transformers T1A to T1D included in the discharge tube lighting circuit 105, respectively. The transformers T1A to T1D are connected to the power supply electrodes so that output voltages of secondary windings N2A and N2B can be opposite in polarity at both ends of the U-shaped discharge tube 102A and output voltages of secondary windings N2C and N2D can be opposite in polarity at both ends of the U-shaped discharge tube 102B. On the primary sides of the transformers T1A to T1D, primary windings N1A to N1D are connected in parallel, and this parallel circuit is connected to a high-frequency driving circuit 110.
The high-frequency driving circuit 110 is an inverter that supplies an AC voltage to the U-shaped discharge tubes 102A and 102B via the transformers T1A to T1D. The transformers T1A to T1D boost the primary voltage in accordance with a turns ratio thereof, and set the boosted voltage as a predetermined secondary output voltage (1 to 2 kV). In each of the U-shaped discharge tubes 102A and 102B, the secondary output voltages of opposite polarity are individually applied to terminals of the U-shaped discharge tube. Thus, the U-shaped discharge tube is driven by a drive voltage of approximately 2 to 4 kV that is obtained by adding amplitudes of the secondary output voltages applied to the terminals thereof.
Thus, voltages of opposite polarity are individually applied to terminals of a U-shaped discharge tube. Accordingly, a drive voltage required for driving the U-shaped discharge tube can be supplied from both terminals thereof to the U-shaped discharge tube in a distributed manner, whereby a rated output voltage of a transformer connected to each of the terminals can be reduced. Furthermore, all terminals of a plurality of U-shaped discharge tubes are individually connected to transformers that are independent of each other. Accordingly, a voltage across each of the U-shaped discharge tubes is set as a predetermined drive voltage therefor so as to cause the U-shaped discharge tube to emit light regardless of lighting of other U-shaped discharge tubes.
When the above-described U-shaped discharge tube emits light at startup (first lighting), a high starting voltage is required. However, a drive voltage for allowing the U-shaped discharge tube to continuously emit light is lower than the starting voltage. Accordingly, a low set-up ratio is set for each transformer, and a capacitor that series-resonates with a leakage inductance of a secondary winding is disposed for each transformer so as to compensate for the undervoltage at startup. The U-shaped discharge tube is started up using characteristics of this series resonance in which a set-up ratio increases around a resonance point.
Recently, with the increase in size of liquid crystal displays, the number of U-shaped discharge tubes disposed in a liquid crystal display has increased. Discharge tube lighting circuits having the above-described configuration require the same number of transformers and resonant capacitors as the number of terminals of U-shaped discharge tubes. Accordingly, with increasing number of U-shaped discharge tubes to be disposed, it is necessary to increase the number of components such as transformers and resonant capacitors.
The increase in the number of components leads to an increase in manufacturing cost, an increase in component cost, an increase in size of apparatuses, and a deterioration in reliability. Accordingly, these problems have to be solved. Furthermore, since the increase in the number of components leads to an increase in footprint for transformers, the number of U-shaped discharge tubes to be disposed is limited.